3 Main Importances of Scheduling
- Effective scheduling means faster movement of goods and services through a facility. This means greater use of assets and hence greater capacity per dollar invested, which, in turn, lowers cost.
- Added capacity, faster throughput, and the related flexibility mean better customer service through faster delivery.
- Good scheduling contributes to realistic commitments, hence dependable delivery.
Objective of Scheduling
The objective of scheduling is to allocate and prioritize demand (generated by either forecasts or customer orders) available facilities.
Begins with the due date by scheduling the final operation first and the other job steps in reverse order.
4 Scheduling Criteria
1. Minimize completion time.
2. Maximize utilization.
3. Minimize work-in-process inventory.
4. Minimize customer waiting time.
Scheduling Process-Focused Facilities
A process-focused facility is a high-variety, low-volume system commonly found in manufacturing and services.
- It is also called an intermittent, or job shop, facility.
- Control files track the actual progress made against the plan for each work order.
Allows operations personnel to manage facility work flows by tracking work added to a work center and its work completed.
Cards that control the amount of work in a work center, aiding input/output control.
- ConWIP is an acronym for constant work-in-process.
- A ConWIP card travels with a job (or batch) through the work center.
- When the job is finished, the card is released and returned to the initial workstation, authorizing the entry of a new batch into the work center.
Gantt Load Chart
Shows the loading and idle times of several departments, machines, or facilities.
- It displays the relative workloads in the system so that the manager knows what adjustments are appropriate.
Gantt Schedule Chart
Used to monitor jobs in progress (and is also used for project scheduling).
- It indicates which jobs are on schedule and which are ahead of or behind schedule.
A specific class of linear programming models that involves assigning tasks or jobs to resources.
- In assignment problems, only one job (or worker) is assigned to one machine (or project)
- The assignment method involves adding and subtracting appropriate numbers in the table to find the lowest opportunity cost for each assignment.
First-Come, First-Served (FCFS)
Jobs are completed in the order in which they arrived.
- FCFS performs about average on most criteria, and it appears fair to customers.
Shortest Processing Time (SPT)
Jobs with the shortest processing times are assigned first.
- SPT is the best technique for minimizing job flow and average number of jobs in the system.
Average Number of Jobs in the System Formula
(Sum of total flow time) / (Total job work (processing) time)
Critical Ratio (CR)
A sequencing rule that is an index number computed by dividing the time remaining until due date by the work time remaining.
- As opposed to the priority rules, the critical ratio is dynamic and easily updated.
- It tends to perform better than FCFS, SPT, EDD, or LPT on the average job-lateness criterion.
- (Due Date - Today's Date) / (Work (lead) time remaining)
An approach that minimizes processing time for sequencing a group of jobs through two work centers while minimizing total idle time in the work centers.
3 Limitations of Rule-Based Scheduling Systems
1. Scheduling is dynamic.
2. Rules do not look upstream or downstream.
3. Rules do not look beyond due dates.
Finite Capacity Scheduling (FCS)
Computerized short-term scheduling that overcomes the disadvantage of rule-based systems by providing the user with graphical interactive computing.
Level Material Use
The use of frequent, high-quality, small lot sizes that contribute to just-in-time production.